Known examples of protein-containing acidic food and drink are acidic milk drinks. These acidic milk drinks are typically prepared by adding sours such as fruit juices, acidic electrolytes and the like to the main component of fermented milk that is obtained through fermentation of milk with lactic acid bacteria or yeasts. However, fermented milk has little taste for refreshment, and, in addition, it coagulates to a rough and non-smooth texture when electrolytes, fats and oils are added thereto.
Known methods for producing smooth, acidic milk drinks, for example, include: a method of homogenizing a raw material mixture that comprises acidic milk, pectin and a calcium-containing component, and a method of homogenizing a raw material mixture that comprises acidic milk and pectin followed by adding calcium thereto (Japanese Unexamined Patent Publication No. 56567/96); and a method of forming fat-containing, condensed sour milk beverage containing lactic acid bacteria through high pressure homogenization, which are characterized by stable fat dispersion therein (Japanese Unexamined Patent Publication No. 20057/89). However, even in these methods, the smoothness of the acidic milk drinks obtained is still unsatisfactory.
On the other hand, long-term storage of acidic milk drinks is difficult. In general, acidic milk drinks can only be distributed in the market at low temperatures and for a period of 2 weeks or so. In order to distribute acidic milk drinks in the market at room temperature for a longer period of time, they must be further treated. For example, they must be sterilized by ultra-high-temperature flash pasteurization (UHT) or retorting. Alternatively, fermented milk is processed with an acid added thereto, thereby having a pH of not higher than 4.0, and thereafter this is subjected to cold pasteurization. In those sterilization methods, however, acidic milk drinks generally coagulate.
On the other hand, known methods for producing acidic milk drinks that are not derived from fermented milk include: a method of adding stabilizers (Japanese Examined Patent Publication No. 35470/73, Japanese Unexamined Patent Publication No. 280366/96, etc.), or a method of adding saccharides (U.S. Pat. No. 3,800,052); and a method of solubilizing protein in milk with strong acids (Japanese Unexamined Patent Publication No. 20508/74), etc.
However, in those methods for producing acidic milk products that do not start with fermented milk, most proteins that are present in the starting milk pass through their own isoelectric point, and coagulate when they pass through the isoelectric point. This means that good products are difficult to produce.
For these reasons, few protein-containing acidic drinks are known other than the acidic milk drinks that are produced from fermented milk.
On the other hand, there is increasing a demand for liquid nutrients such as thick liquid diets that contain protein as the nitrogen source. Many of conventional liquid nutrients that contain protein as the nitrogen source are processed to a pH value that falls within a neutral range. However, the nutrients that are processed to have a pH value that falls within a neutral range are defective in that they often taste oily and their taste is bad. Therefore, it is desirable to develop protein-containing acidic foods and drinks such as acidic liquid nutrients, etc. However, even in the process of producing high-protein, acidic liquid nutrients, the same problems occur as in the process of producing acidic milk drinks not from fermented milk. In particular, when liquid nutrients are made and are supplemented with electrolytes such as minerals, water-soluble vitamins and the like, the coagulation of protein is promoted by the electrolytes when the protein passes through its isoelectric point. Therefore, in that case, good acidic liquid nutrients are difficult to obtain.
On the other hand, as nutrient supplements for aged persons having lowered chewing and swallowing power, protein-containing semi-solid foods such as jellies, puddings, etc. are desirable. However, for the same reasons as those for protein-containing acid drinks noted above, it is difficult to produce acidic, protein-containing semi-solid foods.